Polymer foams are frequently used for filling cavities with foam or for sealing connecting elements. Polyurethane prepolymers are normally used for the production of such foams. Principal applications are building construction and technical products where cavities have to be filled to avoid condensation traps.
Prepolymers for the production of polyurethane foams have crosslinkable polyisocyanate groups which react with a polyol component or water to form the actual polymer. The foam structure is produced by CO2 formed during the reaction with water or by blowing gas present in the prepolymer mixture or by a combination of both elements.
The isocyanate groups present in the prepolymer mixtures for the production of polyurethane foams are highly reactive and are extremely irritative and toxic towards organisms. Where such mixtures are applied with propellant gases from aerosol cans, there is a risk of aerosol formation, particularly in the event of improper handling so that material containing isocyanate groups can pass onto the skin and into the respiratory tract which is undesirable.
Besides relatively high molecular weight prepolymers, the polyisocyanate-based prepolymer mixtures used in pressurized containers for foam generation generally also contain unreacted polyisocyanates or polyisocyanates which have only reacted off to low molecular weight prepolymers. These constituents of the prepolymer mixtures form the actual danger source because of their relatively high volatility and are therefore undesirable. However, apart from this potential hazard, which proper handling can reduce to a low-risk classification, there is an acceptance problem among many users which is increased by the manufacturer's obligation to declare such products as toxic and by the classification of the empty containers as special waste. This applies in particular to the DIY enthusiast who buys his product in DIY stores.
In the past, various measures have been taken to reduce the toxicity of conventional foamable polyisocyanate-based prepolymer mixtures for aerosol cans, more particularly through the choice of certain polyisocyanates for the preparation of the prepolymers and by reducing the percentage content of monomeric constituents in the prepolymer mixtures. Although, overall, these measures have been successful in regard to the release of isocyanates, the acceptance problem has not been satisfactorily solved because the mixtures in question still contain isocyanates.
Processes for the production of polymer foams substantially free from isocyanate groups are known from the prior art. These polymer foams can be dispensed from aerosol cans or pressurized containers of the conventional type, i.e. without the application technique having to be changed. The handling of such polymer foams largely corresponds to the handling of conventional isocyanate-based polymer foams.
Thus, WO-A 00/04069 describes a prepolymer mixture containing silane-terminated prepolymers. The prepolymer compound is a silane-terminated polyurethane polymer containing at least two alkoxysilyl groups. Prepolymers such as these are produced by initially preparing polyurethane prepolymers containing isocyanate groups which are then terminally reacted with alkoxysilanes containing at least one isocyanate-reactive group. The content of silane termini in the prepolymer is at least 5% by weight. However, a problem in this regard is that the reduction in the content of alkoxysilyl groups in the described systems is normally accompanied by a deterioration in the material properties of the foam to the extent that the foam is too soft and not sufficiently tear-resistant for many applications.
Corresponding problems arise where compounds containing alkoxysilyl groups are used as binders in surface coating compositions, for example in paints or adhesives.
JP 63-112605 A (Abstract) relates to a resin with good flexibility, solvent resistance and adhesion to organic materials. The resin may be used as a surface coating composition, adhesive, sealant or primer. A corresponding resin is produced by radical polymerization of compounds containing olefinically unsaturated double bonds, for example styrene or methacrylates, and oligomeric compounds capable of radical polymerization which bear a silyl group. The oligomeric compounds have a molecular weight of 200 to 60,000. The compounds formed have a C-C polymer backbone throughout, the oligomers present during the polymerization being inserted as side chains. A graft polymer with radically or ionically grafted side chains is not mentioned in the cited document.
EP-A 0 392 567 also relates to a resin with good flexibility, solvent resistance and adhesion to organic materials. This resin also may be used as a surface coating, adhesive, sealant or primer. The document in question also mentions grafting as one of the many possible methods of producing the described polymers. However, it does not mention graft polymers containing at least 0.5% by weight of graft branches.
EP-A 0 427 293 describes a curable composition containing a polymer with silyl groups, an organic silyl compound with a molecular weight of less than 3,000 and a compound containing silanol groups. The polymers containing silyl groups described in this document include inter alia compounds obtainable by copolymerization of macromonomers with low molecular weight vinyl monomers. However, the cited document does not mention graft polymers containing at least 0.5% by weight of graft branches.
WO 93/05089 describes polyurethane compounds containing silyl groups. It does not mention copolymerization with olefinically unsaturated monomers.
JP 04-103606 (CP 92-178115/22) relates to reactive isobutylene polymers containing Si groups. The polymers contain structural units with Si-containing side chains which were grafted onto the main polymer by hydrosilylation. However, the cited document does not mention graft polymers containing at least 0.5% by weight of graft branches.
Accordingly, the problem addressed by the present invention was to provide binders and binder compositions containing alkoxysilyl groups which, despite a reduced emission of alcohols during curing, would lead to surface coating materials with favorable properties, for example to adhesives with universal adhesive properties, i.e. with adhesion to a broad range of different substrates, or to rigid foams with high ultimate strength. The last point in particular could not be fulfilled using silylated prepolymers with a low content of alkoxysilane groups because, hitherto, such systems only led to flexible, elastic products.